Electromagnetic signals are sensitive to fluids, particularly, to the specific composition of fluids. For example, saline brines that are sometimes used to produce hydrocarbons are highly conductive and generate a resistivity contrast of one or more orders of magnitude relative to in-situ oil. When saline water displaces oil in the hydrocarbon-bearing formation, such changes can be tracked by measuring the relative variations of the electromagnetic signals taken before and after the saline water has displaced the oil. Because the conditions in the formation change over time, absolute repeatable measurements over long periods of time can be problematic. Repeated measurements over time are also affected by the durability of instrumentation deployed to take such measurements.